Method of degassing melt of light metal



United States Patent ice 3,025,154 METHOD OF DEGASSING MELT OF LIGHT METAL Virgil B. Kurfman, Midland, Mich, assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed Aug. 31, 1959, Ser. No. 836,899

12 Claims. (Cl. 75-67) The invention relates to a method of treating a molten body of light metal to remove occluded gases therefrom and more particularly relates to an improved method of degassing a molten light metal according to a process in which a heavy underlying saline flux is employed.

Molten metals, particularly light metals, often contain dissolved gases. The problem of degassing such melts is unusually difiicult because Henrys law does not apply to solutions of gases in molten metals. A molten metal dissolves an increasing quantity of a gas as the melt temperature is increased. In addition such gas-in-metal solutions tend not to behave reversibly, that is to say, as the molten metal is cooled from an elevated temperature the melt does not give off a gas at the same rate the gas was taken up. And on allowing the gassy melt to solidify 'there is a tendency for a considerable volume of the gas to be evolved abruptly causing blow holes in the solidified metal or if the gas is coalesced only into small bubbles the solidified'metal may simply be porous in structure. Either of these results is undesirable.

Various expedients have heretofore been employed in efforts to rid molten metals of gas content and thus overcome the adverse effects of gas. Melting the charge under a vacuum has been tried and while some success has been had on a laboratory scale in extracting gas from the metal the special equipment required tends to preclude handling of large molten charges. The centrifugal casting of metal has also been proposed for degassing molten metal, but here, too, large scale operations are not practicable.

ther means of degassing have been proposed, such as the addition to the molten metal of chemical compounds. The use of chemical compounds is at best only partially satisfactory in that other problems arise, such as concurrent contamination of the melt. In still other attempts to degas molten metal, both inert and active gases have been introduced directly into the molten metal. But splashing of the molten metal as the gas bubbles break the surface of the melt and resultant atmospheric attack on the splashing metal as well as loss of the metal actually splashing out of the pot or crucible have constituted limitations which have prevented wide adoption of the method. Further, the use of a gas such as chlorine results, in some alloy systems, in grain coarsening which cannot be tolerated in a processing scheme in which the degassing step is the last in the process, for example, following grain refining steps.

It is therefore an object of the invention to provide an improved method of degassing a molten body of light metal.

Another object of the invention is to provide an improved method of degassing a molten body of light metal with a gas which does not result in concurrent melt contamination or grain coarsening or otherwise impair the casting qualities of the melt or the physical properties of solidified metal cast therefrom.

Another object of the invention is to providean improved method of degassing a molten body of light metal with a gas in such a way that violent splashing of the melt does not occur.

Another object of the invention is to provide an improved method of degassing a molten body of light metal 3,025,154 Patented Mar. 13, 1962 which does not require the use of degassing agents which are corrosive to foundry or metal handling equipment.

A further object of the invention is to provide a method of degassing a molten body of light metal which is compatible with known alloying and grain refining procedures.

Still another object of the invention is to provide an improved method of degassing a molten body of light metal which is practical to carry out on large melts.

These and other objects and advantages of the present invention will become apparent upon becoming familiar with the following description and claims.

The invention is predicated upon the discovery that on treating for a time a molten body of a light metal having a molten saline flux layer thereunder with a stream of gas, such as nitrogen, the gas being introduced in such a manner as to sweep up into the melt successive portions of the said underlying flux and the stream of gas bubbling ployed after the melting and alloying of the light metal,

such as magnesium or aluminum, in the presence of a suitable protective saline flux and after such requisite processing steps as flux refining and grain refining have been carried out and the melt is about to be cast.

Suitable fluxes are those which are (1) molten or fluid at the usual melt treating temperatures, (2) possessed of greater density than the melt and (3) capable of at least some flux refining action. An example of a flux which may be used in degassing molten aluminum or aluminum-base alloy is one containing 70 weight percent of barium chloride and 30 weight percent of sodium chloride. 1

An aluminum-base alloy is here defined as an alloy containing at least weight percent of aluminum.

Magnesium and magnesium-base alloys may be treated with one of the following fluxes:

Flux Composition Weight percent 2s MnClz.. 72

A magnesium-base alloy is here defined as an alloy containing at least 75 weight percent of magnesium.

In carrying out the degassing process, the melt to be treated is held over the said heavy underlying fluid saline flux in a suitable vessel such as a pot used in commercial open pot foundry practice. A stream of bubbles of a gas such as nitrogen is introduced into the charge in the vessel via the underlying flux layer by means of a gas delivery pipe or tube. The gas delivery pipe may also be disposed if desired in the molten metal layer with the discharge end located but a short distance, such as two or three inches, above the flux layer and opening downwardly. The gas must then be injected as a stream having sufficient velocity to penetrate into the flux layer. In any event it is essential that as the gas flows from the delivery pipe some of the fluid flux is swept up through the molten metal by the stream of gas bubbles.

Although the mechanism is not clearly understood it is believed that a film of flux surrounds each gas bubble and that as a result gases dissolved or entrained in the melt, for example hydrogen, more easily cross the vapormetal interface than in the absence of the flux. In addition the gas bubbles rising up through the melt break the surface smoothly with substantially no splashing.

Gases which may be employed according to the practice of the invention are those which when passed through a dense fluid saline flux, such as those described above,

do not react with molten light metal when protected therefrom by the fiux. Hydrogen and chlorine may be used if desired but chlorine is generally to be avoided because of its corrosive nature, while hydrogen not only is a rather inefiicient degassing agent but it readily forms explosive mixtures with air as it leaves the melt. Examples of more suitable gases are nitrogen, air, helium, argon, carbon dioxide, carbon monoxide, methane, natural gas, and mixtures thereof. On the other hand a mixture of one of the more suitable gases with up to percent by volume of chlorine but preferably from 1 to 5 percent is advantageously employed to obtain quick gas removal from the melt substantially without concurrent grain coarsening.

. The following examples are illustrative of the practice of the invention:

Example I About 30 pounds of a magnesium-base alloy having the ASTM designation AZ92A and having a nominal composition of 9 weight percent of aluminum, 2 weight percent of zinc and the balance commercial magnesium was melted and brought to a temperature of 1400 F. in a 60 pound capacity steel crucible in the presence of 6 to 8 pounds of No. 230 fiux which consists of, by weight, 55% of KC-l, 34% of MgCI 9% of BaCl and 2% CaF To assure a high gas content in the molten metal, hydrogen, for a minute period, was bubbled directly into the melt in such a way as not to induce any fiux to rise into the metal phase and a flat triangular test panel was cast from the melt. Then nitrogen was bubbled into the melt for 5 minutes by means of a steel delivery pipe inserted into the flux layer in the bottom of the crucible. A second triangular test panel was cast from the so-treated melt. Radiographical examination of the two test panels revealed porosity in the first panel but not in the second.

The mold and test panel in each case were similar to those more fully described by R. S. Busk et al. in the journal article Effect of Gas on the Properties of Magnesium Sand Casting Alloys, published in The American Foundryman, May 1945. The mold is so designed that as increasingly gassy melts are cast, porosity in the test panel will begin to appear at the gate adjacent the base of the triangle or at the base of the triangle and progress toward the apex. The length of porosity-free panel in inches from the apex toward the gate end is a measure of the gas content of the melt.

Example 11 The melt described in Example I was further treated by addition of CaF to inspissate the flux and hydrogen was bubbled directly into the melt in the manner described in Example I for 15 minutes, at a temperature of 1500 F., to again raise the gas content of the melt. A first triangular test panel was cast. Then additional No. 23 0 flux was added to renew the liquid flux layer at the bottom of the crucible and a second triangular test panel was cast. Compressed air was then bubbled into the melt for 15 minutes by means of a steel delivery pipe inserted into the flux layer under the melt while the melt temperature was maintained in the range of 1450 to 1500 F. A third triangular test panel was cast. Radiographioal examination of the test panels showed a porous structure in the first two panels but the third panel was sound.

Example III About 200 pounds of a magnesium-base alloy having the ASTM designation AZ92A was melted and brought to a temperature of 1400 F. in a 300 pound capacity steel crucible in the presence of about pounds of No. 230 flux. Hydrogen was bubbled directly into the melt in the manner described in Example 1 f0! about 15 minutes and the melt was given a conventional carbon grain refining treatment. A triangular test panel was cast from the melt. Nitrogen was then bubbled through the melt for 15 minutes by means of a steel delivery pipe inserted into the flux layer under the melt and a second test panel was cast. Metallographic examination of the test panel showed that the solidified metal in both panels had an average grain diameter of 0.005 inch. Radiographical examination of the test panels also revealed the first one was porous while the second one was free from porosity. The metallic structure of the second test panel was thus both fine grained and nonporous.

Example IV Fifty pounds of a magnesium-base alloy having the ASTM designation AZ63A and having a nominal composition of 6 weight percent of aluminum and 3 weight percent of zinc, the balance commercial magnesium, was melted and brought to a temperature of 1400 F. in a steel crucible in the presence of 5 to 10 pounds of No. 230 flux. Hydrogen was bubbled directly into the melt for 15 minutes in the manner described in Example I. A triangular test panel was cast. Then compressed air, dried by passing it over a mixture of CaCl and Mg(ClO.;) was bubbled into the melt for 30 minutes by means of a steel delivery pipe inserted into the fluid flux layer in the bot-tom of the crucible. A second test panel was cast. One pound of CaC a grain refining agent, was added to the melt and hydrogen was bubbled directly into the melt for 15 minutes while the melt was at a temperature of 1600" F. The melt temperature was lowered to 1400 F. and a third test panel was cast and nitrogen was then bubbled into the melt via the heavy flux layer for 15 minutes and a fourth test panel was cast. All castings were made with a short riser on the mold near the gate to increase the severity of the test. Radiographical examination of the test panels revealed that only one of a possible ten inches of the first panel was free from porosity; six inches of the second panel were sound and the porosity of the remainder of the panel was lightly scattered; all of the third test panel was porous and it even had some holes in it; nine inches of the fourth panel was sound and nonporous in structure.

Example V 56.4 pounds of aluminum was melted and brought to 1600 F. in a graphite crucible and alloyed with 3.6 pounds of ferrosilicon to produce an aluminum alloy containing about 5 percent of silicon. The melt was then heated to 1500 F. and hydrogen was bubbled directly into the melt for 15 minutes in the manner described in Example I. The melt was then cooled to 1350 F. and a triangular test panel was cast.

" bubbled directly into the melt for 15 minutes to assure a high gas content after which the melt was cooled to 1350 F. and a third test panel was cast. A flux consisting of 2.8 pounds of BaCl and 1.2 pounds of NaCl was stirred into the melt to form a heavy fluid flux. Nitrogen was then bubbled through the melt for 30 minutes by means of a steel delivery pipe inserted into the underlying fluid flux. A fourth test panel was then cast. In each of the four castings carried out in this series, the triangular mold was equipped with a short riser near the gate at the base of the triangle. Radiographical examination of the test panels showed that the first and third panels were very porous throughout; the second panel cast, after treating the melt in the absence of a heavy flux, showed scattered pinholes; but the fourth panel was completely sound.

By way of an additional comparison test or blank, about 50 pounds of a magnesium-base alloy having the ASTM designation AZ92A was melted and brought to 1400 F.

in a 60 pound capacity steel crucible and hydrogen was bubbled directly into the melt for 15 minutes in the manner described in Example I. Conventional crucible practice was followed using a dry No. 310 flux consisting of 50 weight percent of MgCl 20 weight percent of KCl, 15 weight percent of MgO and 15 weight percent of CaF No fluid flux layer was present in the bottom of the crucible. The melt was bubbled with nitrogen for 4 minutes by means of a steel delivery pipe extending near the bottom of the crucible and a triangular test panel was cast.

Then nitrogen was bubbled through the melt via the steel delivery pipe for an additional 15 minutes and a second test panel was cast. Radiography of both test panels indicated both were quite porous.

Among the advantages of the invention are the stirring effects which permit concurrent grain refining of the melt, the flexibility in choice of grain refining agent, and the simultaneous flux refining action obtained.

What is claimed is:

l. The method of treating a molten body of light metal containing an occluded gas as an impurity which comprises maintaining under and in contact with the body of molten light metal a quantity of a fluid saline flux therefor, having a greater density than the said molten light metal, and dispersing a portion of said saline flux through the molten light metal by means of a stream of a gas which is substantially inert towards the melt when insulated therefrom by a film of said fluid flux, said stream of gas being passed through the flux and then through the molten light metal so as to form rising gas bubbles surrounded by insulating flux films in an amount and for a time sulficient to substantially degas said molten light metal.

2. The method as in claim 1 in which the light metal is selected from aluminum and aluminum-base alloys.

3. The method as in claim 1 in which the light metal is selected from magnesium and magnesium-base alloys.

4. The method as in claim 1 in which the gas which is substantially inert towards the melt is composed predominantly of a gas selected from the group consisting of nitrogen, air, helium, argon, carbon dioxide, carbon monoxide, methane, natural gas, and mixtures thereof.

5. The method as in claim 4 in which the said gas contains up to 10 percent by volume of chlorine.

6. The method as in claim 5 in which the light metal is selected from aluminum and aluminum-base alloys.

7. The method as in claim 5 in which the light metal is selected from magnesium and magnesium-base alloys.

8. The method as in claim 5 in which the gas, which is substantially inert towards the melt when insulated therefrom by a film of said fluid flux, is composed predominantly of a gas selected from the group consisting of nitrogen, air, helium, argon, carbon dioxide, carbon monoxide, methane, natural gas and mixtures thereof.

. 9. The method as in claim 8 in which the said gas contains up to 10 percent by volume of chlorine.

10. The method of treating a molten body of light metal containing an occluded gas as an impurity which comprises maintaining under and in contact with the body of molten light metal a quantity of a fluid saline flux therefor having a greater density than the said molten light metal and passing a stream of a gas, which is substantially inert towards the melt when insulated therefrom by a film of said fluid flux, into the flux layer under the molten light metal and through the molten light metal so as to form rising gas bubbles surrounded by insulating flux films in amount and for a time sufiicient to substantially degas said molten light metal.

11. In the method of degassing a molten body of light metal by bubbling a gas therethrough the improvement which comprises maintaining under and in contact with the body of molten light metal a quantity of a fluid saline flux therefor, having a greater density than the said molten light metal, and dispersing a portion of said saline flux through the molten light metal by means of a stream of a gas which is substantially inert towards the melt when insulated therefrom by a film of said fluid flux, said stream of gas being passed through the flux and then through the molten light metal so as to form rising gas bubbles surrounded by insulating flux films in an amount and for a time sufiicient to substantially degas said molten light metal.

12. In the method of degassing a molten body of light metal by bubbling a gas therethrough the improvement which comprises maintaining under and in contact with the body of molten light metal a quantity of a fluid saline flux therefor having a greater density than the said molten light metal and passing a stream of a gas, which is substantially inert towards the melt when insulated therefrom by a film of said fluid flux, into the flux layer under the molten light metal and through the molten light metal so as to form rising gas bubbles surrounded by insulating flux films in amount and for a time sufficient to substantially degas said molten light metal.

References Cited in the file of this patent UNITED STATES PATENTS 2,826,489 Wagner Mar. 11, 1958 FOREIGN PATENTS 530,488 Canada Sept. 18, 1956 OTHER REFERENCES Eastwood: Gas in Light Alloys; publisher, John Wiley and Sons, New York, 1946, pp. 77 through 86. 

11. IN THE METHOD OF DEGASSING A MOLTEN BODY OF LIGHT METAL BY BUBBLING A GAS THERETHROUGH THE IMPROVEMENT WHICH COMPRISES MAINTAINING UNDER AND IN CONTACT WITH THE BODY OF MOLTEN LIGHT METAL A QUANTITY OF A FLUID SALINE FLUX THEREFOR, HAVING A GREATER DENSITY THAN THE SAID MOLTEN LIGHT METAL, AND DISPERSING A PORTION OF SAID SALINE FLUX THOUGH THE MOLTEN LIGHT METAL BY MEANS OF A STREAM OF A GAS WHICH IS SUBSTANTIALLY INERT TOWARDS THE MELT WHEN INSULATED THEREFROM BY A FILM OF SAID FLUID FLUX, SAID STREAM OF GAS BEING PASSED THROUGHT THE FLUX AND THEN THROUGH THE MOLTEN LIGHT METAL SO AS TO FORM RISING GAS BUBBLES SURROUNDED BY INSULATING FLUX FLIMS IN AN AMOUNT AND FOR A TIME SUFFICIENT TO SUBSTANTIALLY DEGAS SAID MOLTEN LIGHT METAL. 